The present invention relates to magneto-optical disks, tapes, cards, and other similar storage media used in conjunction with magneto-optical recording/reproduction devices, and their reproduction methods.
Conventionally, magneto-optical storage media have been commercialized as rewritable optical storage media. Such a magneto-optical storage medium has a disadvantage that reproduction characteristics deteriorate when the diameters of, and intervals between, recording bits, i.e., recording magnetic domains, are reduced too much relative to the diameter of a spot formed on the magneto-optical storage medium by focusing a light beam projected by a semiconductor laser device.
This is because the light beam focused on the targeted recording bit encompasses adjacent recording bits within its coverage and fails to separately reproduce the individual recording bits.
To overcome the disadvantage, various magnetic super high resolution reproduction technologies have been developed using a magnetic multi-layer film. These magnetic super high resolution reproduction technologies reduce signal interference during reproduction by forming a magnetic masking area and thus forming a magnetic aperture smaller than the beam spot, thus enabling reproduction of signals whose cycles do not exceed diffraction limits of light.
Nevertheless, the magnetic super high resolution reproduction technologies have a problem that the strength of reproduced signals decreases with a decrease in the recording cycle for the magnetic recording domain, because the aperture also needs to be reduced in size.
To solve the problem, a method is suggested to enable magnetic domain expansion reproduction without applying a.c. external magnetic fields ((Magnetic Domain Expansion Readout with DC Lasers and DC Magnetic Fields [Magnetic Amplifying Magneto-Optical system, or MAMMOS]), an article from resumes for lectures in 44th Conference organized in spring 1997 by the Society of Applied Physics Researchers, 30a-NF-3, page 1068).
Now, referring to FIG. 17 through FIG. 19, a magneto-optical storage medium based on the foregoing method will be explained. FIGS. 17 and 18 are plan and cross-sectional views schematically illustrating magnetization of the magneto-optical storage medium during reproduction. FIG. 19 is a cross-sectional view showing the arrangement of a magneto-optical disk that is an application of the magneto-optical storage medium.
As shown in FIG. 18, the magneto-optical storage medium has a stack structure including a reproduction layer 201, a supplementary reproduction layer 203, and a storage layer 207. The reproduction layer 201 and the supplementary reproduction layer 202 exhibit in-plane magnetization at room temperature, and changes to perpendicular magnetization when temperature is elevated by projection of a focused light beam 208 (light beam spot 208xe2x80x2 in FIG. 17). The storage layer 204 is constituted by a perpendicularly magnetized film, where magnetic information is stored in the form of directions of the magnetization in magnetic domains 209 and 210.
The reproduction layer 201 is specified to change to perpendicular magnetization at a lower temperature than the supplementary reproduction layer 203 changes to perpendicular magnetization. Consequently, on heating by the light beam 208, the magnetic domain 212 in which the reproduction layer 201 has changed to perpendicular magnetization grows larger than the magnetic domain 211 in which the supplementary reproduction layer 203 changes to perpendicular magnetization.
The direction of the magnetization in the magnetic domain 211, in which the supplementary reproduction layer 203 has changed to perpendicular magnetization due to the heat of the light beam 208, is determined by coupling to the storage layer 207 through exchange forces. Hence, the magnetic information in the storage layer 207 is duplicated to the supplementary reproduction layer 203 so that the direction of the auxiliary grating moment of the supplementary reproduction layer 203 conforms to that of the storage layer 207.
Next, the magnetic information in the magnetic domain 211, in which the supplementary reproduction layer 203 has changed to perpendicular magnetization, is duplicated to the reproduction layer 201 so that the direction of the transition metal (TM) moment of the reproduction layer 201 conforms to that of the supplementary reproduction layer 203. Here, since the magnetic domain 212, in which the reproduction layer 201 changes to perpendicular magnetization, grows larger than the magnetic domain 211, in which the supplementary reproduction layer 203 changes to perpendicular magnetization, the magnetization of the supplementary reproduction layer 203, i.e., the magnetization of the storage layer 207, is expanded and duplicated to the reproduction layer 201.
As described above, in the magneto-optical storage medium used in accordance with the aforementioned method, small magnetic domains in the storage layer 207 are expanded and duplicated to the reproduction layer 201; therefore, high density storage is realized, and expansion of reproduction signals is enabled.
It should be noted that as shown in FIG. 19, typically, the foregoing magneto-optical storage medium includes the arrangement shown in FIG. 18, and further constitutes overlapping layers including a substrate 213, a transparent dielectric protection layer 214, and a protection layer 215 among others.
However, the storage layer 207, the supplementary reproduction layer 203, and the reproduction layer 201 are coupled together through exchange forces in the magneto-optical storage medium capable of reproducing magnetic domains by means of expansion in accordance with the aforementioned method.
Therefore, the transition from in-plane to perpendicular magnetization of the supplementary reproduction layer 203 and the reproduction layer 201 proceeds gradually with rising temperature, resulting in difficulties in improving reproduction resolution.
Further, in a vicinity of transition temperature Tp201 at which the reproduction layer 201 changes to perpendicular magnetization, the supplementary reproduction layer 203 exhibits in-plane magnetization and is coupled to the reproduction layer 201 through exchange forces. The coupling interrupts the change of the reproduction layer 201 to perpendicular magnetization, ostensibly raising transition temperature Tp201. Consequently, the magnetic domain 212 formed in the reproduction layer 201 becomes smaller than when no coupling is established through exchange forces. In a vicinity of transition temperature Tp203 at which the supplementary reproduction layer 203 changes to perpendicular magnetization, the reproduction layer 201 exhibits perpendicular magnetization and is coupled to the supplementary reproduction layer 203 through exchange forces. The coupling causes the supplementary reproduction layer 203 to start changing to perpendicular magnetization at a temperature below transition temperature Tp203 at which the supplementary reproduction layer 203 desirably changes to perpendicular magnetization, ostensibly lowering transition temperature Tp203. Consequently, the magnetic domain 211 formed in the supplementary reproduction layer 203 becomes smaller than when no coupling is established through exchange forces.
When the magnetization in the storage layer 207 is duplicated to the supplementary reproduction layer 203, the magnetic domain 211 is larger than the magnetic recording domain 209, and therefore affected by magnetic domains surrounding the magnetic recording domain 209, making it difficult to duplicate the magnetization to the supplementary reproduction layer 203 with high resolution. Further, the magnetization in the magnetic domain 211 in the supplementary reproduction layer is not sufficiently expanded and duplicated to the magnetic domain 212, reducing the strength in reproduction signals and failing to deliver satisfactory signal quality, which is a problem.
Accordingly, so that the change of the supplementary reproduction layer 203 and the reproduction layer 201 from in-plane magnetization to perpendicular magnetization takes place with rising temperature in a stable manner, the supplementary reproduction layer 203 and the reproduction layer 201 need to be thick; however, greater thicknesses of the layers degrades playback sensitivity, which is yet another problem.
The present invention has an object to offer a magneto-optical storage medium that can reproduce signals whose cycles do not exceed diffraction limits of light, with improved signal amplitudes and satisfactory playback sensitivity.
In order to achieve the object, a magneto-optical storage medium includes:
a reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp1;
an in-plane magnetized layer constituted by an in-plane magnetized film having a Curie temperature Tc2 around the transition temperature Tp1;
a storage layer constituted by a perpendicularly magnetized film for storing information; and
at least one supplementary reproduction section, interposed between the storage layer and the in-plane magnetized layer, constituted by a first supplementary reproduction layer and a first in-plane magnetized supplementary reproduction layer, the first supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp3, the first in-plane magnetized supplementary reproduction layer being disposed adjacent the first supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature around the transition temperature Tp3,
wherein Tp1 less than Tp3.
In the arrangement, there is provided an in-plane magnetized layer losing its magnetization at its Curie temperature Tc2 around transition temperature Tp1 at which the reproduction layer changes to perpendicular magnetization. Thus, the in-plane magnetized layer aligns the magnetization of the reproduction layer in the in-plane direction through exchange coupling below Tp1, enhancing in-plane magnetization masking. Above Tp1, the in-plane magnetized layer allows passage to the leaking magnetic field generated in the storage layer and the first supplementary reproduction layer.
Further, there is provided a first in-plane magnetized supplementary reproduction layer losing its magnetization at its Curie temperature Tc4 around transition temperature Tp3 at which the first supplementary reproduction layer changes to perpendicular magnetization. Thus, the first in-plane magnetized supplementary reproduction layer aligns the magnetization of the reproduction layer in the in-plane direction through exchange coupling below Tp3, enhancing in-plane magnetization masking. Above Tp3, the first in-plane magnetized supplementary reproduction layer allows passage to the leaking magnetic field generated in the storage layer.
According to the arrangement, the reproduction layer, the in-plane magnetized layer, the first supplementary reproduction layer, and the first in-plane magnetized supplementary reproduction layer are securely coupled through exchange forces below transition temperature Tp1 at which the reproduction layer changes to perpendicular magnetization, thereby stabilizing the in-plane magnetization of the reproduction layer below transition temperature Tp1. Thus, the reproduction layer abruptly changes from in-plane magnetization to perpendicular magnetization where it is heated exceeding transition temperature Tp1, improving reproduction resolution and enabling stable magnetic domain expansion and duplication.
The exchange coupling between the reproduction layer and the first supplementary reproduction layer is blocked by the in-plane magnetized layer, preventing transition temperature Tp1 of the reproduction layer from rising and transition temperature Tp3 of the first supplementary reproduction layer from falling.
Further, the first supplementary reproduction layer is securely coupled to the in-plane magnetized supplementary reproduction layer through exchange forces below transition temperature Tp3 at which the first supplementary reproduction layer changes to perpendicular magnetization, thereby exhibiting in-plane magnetization with increased stability below transition temperature Tp3 of the first supplementary reproduction layer. Thus, the first supplementary reproduction layer abruptly changes from in-plane magnetization to perpendicular magnetization where it is heated exceeding transition temperature Tp3, improving reproduction resolution in the first supplementary reproduction layer and effecting magnetic domain expansion and duplication with increased stability.
In this manner, high reproduction resolution and satisfactory reproduction signal quality are obtainable by expanding and duplicating, to the reproduction layer, magnetic domains duplicated to the first supplementary reproduction layer with high reproduction resolution.
The supplementary reproduction section may be provided in plurality between the in-plane magnetized layer and the storage layer, in ascending order of the transition temperatures from the in-plane magnetized layer toward the storage layer.
Such an arrangement is realized by the supplementary reproduction section constituted by a first supplementary reproduction section and a second supplementary reproduction section,
the first supplementary reproduction section being constituted by a first supplementary reproduction layer and a first in-plane magnetized supplementary reproduction layer disposed so that the first supplementary reproduction layer is closer to the reproduction layer than is the first in-plane magnetized supplementary reproduction layer, the first supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp3, the first in-plane magnetized supplementary reproduction layer being disposed adjacent the first supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature Tc4 around the transition temperature Tp3,
the second supplementary reproduction section being constituted by a second supplementary reproduction layer and a second in-plane magnetized supplementary reproduction layer disposed so that the second supplementary reproduction layer is closer to the reproduction layer than is the second in-plane magnetized supplementary reproduction layer, the second supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp5, the second in-plane magnetized supplementary reproduction layer being disposed adjacent the second supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature Tc5 around the transition temperature Tp5,
wherein
the first supplementary reproduction layer is closer to the storage layer than is the second supplementary reproduction layer, and
Tp1 less than Tp5 less than Tp3.
In the arrangement, the magnetic recording domain formed through duplication from the storage layer to the first supplementary reproduction layer is expanded and duplicated to the reproduction layer by expanding and duplicating the magnetic domain sequentially from the first supplementary reproduction layer via the second supplementary reproduction layer to the reproduction layer, effecting smooth expansion and duplication. Besides, a leaking magnetic flux arising in parallel to the total magnetization of the magnetic recording domain from an increased area can be applied to a part of the reproduction layer where it has changed to perpendicular magnetization; therefore, the expansion and duplication to the reproduction layer is more stable than at short mark lengths, as well as external disturbances, such as a leaking magnetic field from the optical pickup head, are less likely to cause negative effects.
Further, the magnetic domain, expanded and duplicated to the second supplementary reproduction layer with high reproduction resolution, is expanded and duplicated to the reproduction layer, producing better reproduction signal quality. Consequently, the magneto-optical storage medium improves reproduction resolution in the second supplementary reproduction layer and achieves satisfactory reproduction signal quality at short mark lengths, while retaining satisfactory recording sensitivity.
In order to achieve the object, a method of reproducing a magneto-optical storage medium is a method of reproducing such a magneto-optical storage medium that includes:
a reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp1;
an in-plane magnetized layer constituted by an in-plane magnetized film having a Curie temperature Tc2 around the transition temperature Tp1;
a storage layer constituted by a perpendicularly magnetized film for storing information; and
at least one supplementary reproduction section, interposed between the storage layer and the in-plane magnetized layer, constituted by a first supplementary reproduction layer and a first in-plane magnetized supplementary reproduction layer, the first supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp3, the first in-plane magnetized supplementary reproduction layer being disposed adjacent the first supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature around the transition temperature Tp3,
wherein Tp1 less than Tp3,
the method being characterized in that it includes the steps of:
(a) heating the magneto-optical storage medium exceeding the transition temperature Tp3 by means of a light beam for reproduction;
(b) duplicating the magnetic information stored in the storage layer to a part of the first supplementary reproduction layer where the first supplementary reproduction layer is hotter than the transition temperature Tp3; and
(c) duplicating the magnetic information duplicated to the part of the first supplementary reproduction layer to a part of the reproduction layer where the reproduction layer is hotter than the transition temperature Tp1.
In order to achieve the object, another method of reproducing a magneto-optical storage medium is a method of reproducing such a magneto-optical storage medium that includes:
a reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp1;
an in-plane magnetized layer constituted by an in-plane magnetized film having a Curie temperature Tc2 around the transition temperature Tp1;
a storage layer constituted by a perpendicularly magnetized film for storing information;
a first supplementary reproduction section being constituted by a first supplementary reproduction layer and a first in-plane magnetized supplementary reproduction layer disposed so that the first supplementary reproduction layer is closer to the reproduction layer than is the first in-plane magnetized supplementary reproduction layer, the first supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp3, the first in-plane magnetized supplementary reproduction layer being disposed adjacent the first supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature Tc4 around the transition temperature Tp3; and
a second supplementary reproduction section being constituted by a second supplementary reproduction layer and a second in-plane magnetized supplementary reproduction layer disposed so that the second supplementary reproduction layer is closer to the reproduction layer than is the second in-plane magnetized supplementary reproduction layer, the second supplementary reproduction layer exhibiting in-plane magnetization at room temperature and changing to perpendicular magnetization above a transition temperature Tp5, the second in-plane magnetized supplementary reproduction layer being disposed adjacent the second supplementary reproduction layer and constituted by an in-plane magnetized film having a Curie temperature Tc5 around the transition temperature Tp5,
wherein
the first supplementary reproduction layer is closer to the storage layer than is the second supplementary reproduction layer, and
Tp1 less than Tp5 less than Tp3
the method being characterized in that it includes the steps of:
(a) heating the magneto-optical storage medium exceeding the transition temperature Tp3 by means of a light beam for reproduction;
(b) duplicating the magnetic information stored in the storage layer to a part of the first supplementary reproduction layer where the first supplementary reproduction layer is hotter than the transition temperature Tp3; and
(c) duplicating the magnetic information duplicated to the part of the first supplementary reproduction layer to a part of the second supplementary reproduction layer where the second supplementary reproduction layer is hotter than the transition temperature Tp5; and
(d) duplicating the magnetic information duplicated to the part of the second supplementary reproduction layer to a part of the reproduction layer where the reproduction layer is hotter than the transition temperature Tp1.
According to either one of the methods of reproducing a magneto-optical storage medium, the magnetic domain in the storage layer is expanded and duplicated to the supplementary reproduction layer and then further expanded and duplicated to the reproduction layer; a leaking magnetic flux arising from an increased area can therefore be applied to the reproduction layer, which improves reproduction signal quality.